1. Field of the Invention
This invention relates generally to anti-reflective coatings for use in deep ultraviolet microlithography, and more particularly to forming such anti-reflective coatings from polymeric materials having intrinsic light-absorbing properties.
2. Description of the Prior Art
Integrated circuit production relies on the use of photolithographic processes to define the active elements and interconnecting structures within devices. Until recently, G-line (436 nm) photoresists have been used for the bulk of microlithographic applications. There is now a strong trend toward the use of resists which are sensitive at shorter wavelengths to improve resolution at submicron feature sizes. This has led to the development of photoresist products which are sensitive to deep ultraviolet (DUV) radiation. The majority of these new resists are designed to operate at 248 nn.
Decreasing feature size coupled with the more widespread use of monochromatic DUV exposure systems has intensified the problems associated with standing wave effects which occur when light is reflected from the substrate during exposure. The formation of standing waves reduces critical dimension control and causes large linewidth variations over device topography.
In accordance with the prior art, several techniques have been used to minimize problems associated with standing wave effects. One such technique which is discussed in C. Nolscher et al., "High Contrast Single Layer Resist and Anti-reflective Layers--An Alternative to Multilayer Resist Techniques," SPIE volume 1086 (1989). In accordance with Nolscher, a carefully controlled thickness of titanium nitride is deposited on the substrate to act as an interference filter for reflected light. However, this technique is not effective for DUV microlithography because of the high reflectivity of most substrates at deep ultraviolet wavelengths.
Another prior art technique for eliminating reflectivity problems involves applying an organic film with a low index of refraction (typically 1.4 or less) over the photoresist to damp out higher order standing waves. This method is discussed in T. Tanaka et al., J. Electrochem. Soc., volume 137, p. 3900 (1990). However, the benefits of this method have not yet been proven for DUV microlithography.
A third prior art technique involves using an anti-reflective layer (referred to hereafter as ARC, meaning anti-reflective coating) beneath the photoresist to inhibit the formation of standing waves and prevent their detrimental effects on photoresist resolution. Numerous anti-reflective layer technologies exist today for use in microlithographic processes. The most common technique is applying a light absorbing organic polymer coating on the substrate prior to applying the photoresist. This method is discussed in M. Listvan et al., "Multiple Layer Techniques in Optical Lithography: Applications to Fine Line MOS Production," SPIE volume 470, p. 85 (1984); U.S. Pat. No. 4,910,122 entitled "Anti-Reflective Coating" issued to T. Brewer; and A. Jeffries et al., "Two Anit-Reflective Coatings for Use Over Highly Reflective Topography," SPIE Proceedings, Volume 539, p. 342 (1985). This method provides nearly complete attenuation of reflected light and prevents formation of all standing waves including the primary, or first-order, standing wave. The location of the light absorbing layer between the substrate and resist means that all substrates coated with the material will exhibit roughly the same reflectivity, eliminating the need for exposure dose adjustments when the substrate type changes. This prior art technique offers the greatest control over standing wave effects and at the same time provides the widest photoresist processing latitude and broadest substrate compatibility.
Traditionally, ARCs have been formulated by adding dyes which absorb at the exposure wavelength to a polymer coating. While a series of prior art dye-loaded commercial products have been prepared, dye-loaded ARCs are susceptible to numerous problems. One such problem involves separation of the polymer and dye components when the ARC is spin coated. This phenomenon is evidenced by the appearance of film defects such as graininess, blotches, spots, dewerred areas, etc., and in some cases complete coating failure.
Problems also result from variations in the dye content of the ARC which depend on spinning conditions. For example, we have observed that dye compounds can be removed selectively from the wet ARC as excess coating material spins off of the substrate. This problem is especially prevalent with crystalline dyes.
A third problem with dye-loaded ARCs involves dissolution or swelling of the anti-reflective coating accompanied by selective removal of the dye components. This problem is commonly referred to as dye stripping. The dye stripped from the ARC remains in the photoresist and causes nonuniformities in the resist exposure profile. This is especially true with DUV photoresists which operate via acid catalysis and are easily poisoned by organic contaminants. At the same time, dye loss by stripping reduces the ability of the ARC to control reflections. Dye stripping depends on the properties of the dye and polymer components as well as their relative proportions and chemical interactions.
Still another problem with dye-loaded ARCs is interfacial layer formation. This refers to a situation where the components of the resist and the ARC become mixed in a narrow zone between the two layers. Interfacial layer formation invariably leads to "resist footing" which refers to the presence of a small protrusion at the bottom of the resist feature after development. This defect complicates linewidth determination and reduces critical dimension control.
There is also a problem with thermal diffusion of the dye component into the photoresist when the latter is baked. Dye diffusion can occur over distances as large as 50%-60% of the photoresist thickness. In such cases, photoresist contrast and sidewall profiles will be seriously degraded.
These problems are particularly prevalent with prior art dye loaded ARCs designed for DUV microlithography. DUV photoresists contain more aggressive solvents than conventional G-line resist systems. Such solvents enhance dye stripping and interfacial layer formation. In some cases, complete removal of an ARC has been observed when a DUV resist is applied. Furthermore, in formulating dye loaded DUV ARCs the majority of dyes suitable for this spectral region are either 1) very soluble in photoresist solvents or 2) highly crystalline, meaning in either case that the dye concentration in the ARC must be kept relatively low. As a result, the ARC thickness must be increased to.gtoreq.2000 .ANG. to obtain sufficient light absorptivity with the film.
The necessity for ARC thickness of.gtoreq.2000 .ANG. seriously inhibits the development of ARCs for submicron DUV microlithographic processing. For submicron work, anisotropic dry etching (plasma etching or reactive ion etching) must be used to transfer the resist pattern into the ARC. In turn, the ARC must be ultra-thin (.ltoreq.1300 .ANG.) to limit the amount of resist erosion that will occur when the ARC is dry etched and to maintain an acceptable etch bias. However, the limits on dye concentration which are imposed by various chemical factors make the ultra-thin thickness goal unattainable. For this reason, the development of dye-loaded ARCs for DUV microlithography has been severely curtailed.
Dye-loading is not amenable to the development of ARCs for deep ultraviolet microlithography. For some of the same reasons, the polymeric materials used in prior art ARCs are unsuitable for use in DUV anti-reflective coating products. The table below provides several relevant examples.
__________________________________________________________________________ Limitations for Material Use in DUV ARCs References __________________________________________________________________________ polymethyl methacrylate excessive interlayer mixing U.S. Pat. No. 4,370,405 to with resist O'Toole; and A. Yen et al, "Fabrication of 100 nm- Period Gratings Using Achromatic Holographic Lithograph," Microelectronic Engineering, Vol. 11, p. 201 (1990). polyimides - poor solvent resistance U.S. Pat. No. 4,910,122 to T. soluble polyimides and/or poor coating quality Brewer; M. W. Legenza et and may be soluble in al, "A New Class of Bilevel photoresist developers and Mono-level Positive Resist Systems Based on a Chemically Stable Imide curable polyimides unstable solution viscosity Polymer," SPIE Proceedings, leading to poor film Volume 539, p. 250 (1985); thickness reproducibility, and C. H. Ting and K. L. must be refrigerated Liauw, "An improved Deep Ultraviolet (DUV) Multlayer Resist Process for High Resolution Lithography," SPIE Proceedings, Volume 469, p. 24 (1984). polyvinylpyridine low DUV light absorption, poor solvent resistance triazine resins soluble in photoresist W. Ishii, et al, "Anti- developers Reflective Coating Material for Highly Reflective Surfaces with Topography", SPIE volume 631, p. 295 (1986). poly(alkene sulfones) poor solvent resistance and U.S. Pat. No. 4,910,122 to T. low thermal stability Brewer. __________________________________________________________________________
Another group of polymeric materials which have been used for microlithographic applications are the aromatic polysulfones. For example, U.S. Pat. No. 4,835,086 describes the use of Udel.sup.R (Amoco Performance Products, Inc.) polysulfone as a barrier layer between two radiation sensitive layers. Another polysulfone, Victrex.sup.R (ICI Americas), has been preferred for most microlithographic applications, e.g., lift-off processes, because of its higher solvent resistance in comparison to Udel.sup.R. The structure for Victrex.RTM. polysulfone is: ##STR1## Victrex.sup.R must be spin coated from N-methylpyrrolidone, a strong polar solvent since it is insoluble in less polar, but more volatile solvents such as ketones or esters. European Patent Application number 8710976.0 (publication number 0 257 255) describes the use of Victrex.sup.R films containing dyes to retard the detrimental effects of reflected light during photoresist exposure. However, such films do not have the appropriate solubility properties for ARC applications. This can be explained as follows.
There are two main types of DUV photoresists in use today, positive-working and negative-working. Both operate via acid catalysis. The positive-working type has extremely high contrast and is easily poisoned by small amounts of bases, especially organic amines. While N-methylpyrrolidone is only very weakly basic, it usually contains strongly basic contaminants such as methyl amine at low levels. The volatility of these contaminants and their detrimental effect on resist performance precludes the use of NMP-containing materials such as a Victrex.RTM. coating and a positive DUV resist in the same work area.
Negative-working DUV photoresists become insolubilized upon exposure to 248 nm radiation, forming a negative-tone image. Like most negative-working systems, they are more sensitive than positive-tone systems and, consequently, are not susceptible to poisoning by trace levels of basic compounds. However, negative-working DUV resists require more polar solvent systems than their positive-tone counterparts. Diglyme (2-methoxyethyl ether) and anisole, two chemically inert but very polar solvents, are common constituents in the resist formulations; Shipley's SNR-248.RTM. negative-tone DUV photoresist is a good example. The presence of these aggressive solvents in the resist means that the ARC must have exceptional solvent resistance to avoid interlayer mixing. Victrex.RTM. coatings do not have this degree of solvent resistance. Consequently, they cannot be used in ARC applications involving modern negative-working DUV photoresists.